Process and device for the determination of the characteristics of the geological formations traversed by a borehole

ABSTRACT

Process for determining characteristics of the geological formations traversed by a borehole, making use of a sonde by means of which the intensity of the γ rays naturally radiated from the formations is measured, both longitudinal and transversal acoustic waves are transmitted to the formations, their travel time and their attenuation between two receivers placed in contact with the formations are measured and by combination of the so-obtained values a resulting value is elaborated which is representative of one characteristic of the surveyed formations.

This application is a continuation-in-part of Ser. No. 129,534 filedMar. 30, 1971 and now abandoned.

The present invention relates to a process and to a device fordetermining the characteristics of the geological formations traversedby a borehole, these characteristics consisting particularly of thelithology, and possibly .Iadd.other petrophysical characteristics suchas .Iaddend.the porosity and permeability of the formations.

Within the framework of the following description, the lithology of thegeological formations is intended to include the following data:

the clay content (i.e., the entirety of the argillaceous mineralsexpressed in percent;

the carbonate content (calcite, dolomite, aragonite, siderite),expressed in percent;

the quartz content expressed in percent;

the cationic exchange capacity (or CEC), expressed in milliequivalentper 100 grams of rock. (This value is representative of the behavior ofthe clays of the formation being subjected to a change in the ioniccomposition of the fluids which impregnated the same, and due forexample to the drilling.)

It is an object of the present invention to determine the lithology ofgeological formations traversed by a borehole on the basis ofmeasurements carried out by so-called radioactive logging processes,such as the γ logging.

The method according to the present invention for determiningcharacteristics of the geological formations traversed by a borehole,using a measuring sonde in combination with an automatic system fortreating the data furnished by the sonde, is characterized in that itcomprises in combination the following steps:

a. determining of the natural γ radiation spectrum emitted by thegeological formations in the form of a plurality of quantities relatedto this radiation, and each of which is a function of the intensity ofthis natural radiation in a given energy band, and

b. elaborating on the basis of these quantities by means of theaforementioned automatic system at least one resultant quantityrepresentative of one characteristic of these formations, this resultantquantity being obtained by a linear combination of these quantitiesrelated to the γ radiation.

These quantities related to the γ radiation will, for example, beelectric signals and the aforementioned resultant quantity will be anelectric signal representative of the lithology of the geologicalformations such as defined previously.

The device for carrying out the process of the present invention, usinga sonde in combination with a surface apparatus for the automatictreatment of the data furnished by the sonde, the sonde and theapparatus being connected by a cable equipped with electricalconductors, is characterized in that the sonde comprises means fordetecting the natural γ radiation emitted by the geological formations,and these means deliver electric signals related to the γ radiationemitted by the geological formations, and each of which is a function ofthe intensity of this natural radiation in a given energy band, andmeans for transmitting these electric signals to the surface.

It is another object of the present invention to simultaneouslydetermine an entire group of characteristics of the geologicalformations comprising, in addition to the lithology, the porosity, andthe permeability of the geological formations traversed by a borehole.

It is possible to determine certain characteristics of the formationstraversed by a borehole by using known so-called radioactive loggingprocesses, such as the γ logging, or so-called acoustic loggings.

In the acoustic logging processes according to the prior art, acousticwave trains are emitted, and if it is desired to distinguish between thelongitudinal and transverse modes of propagation of these acousticwaves, it is necessary to carry out special treatments of the datacollected with the aid of complex apparatus with the obtained resultsbeing of doubtful accuracy.

Moreover, in order to determine the characteristics of formationstraversed by a borehole, it is often necessary, according to the priorart, to establish several loggings for each of the known processes, eachof these loggings requiring a rather significant amount of time to beworked out, and the drilling operation being stopped during the periodsof measurement which thus represent a nonnegligible loss of time andthen it is necessary to make a critical evaluation of all the resultsobtained by these loggings.

The process according to the present invention for overcoming theabove-mentioned drawbacks and for simultaneously determining an entiregroup of characteristics of geological formations traversed by aborehole by using in combination a measuring sonde and an automaticsystem for treating the data furnished by the sonde. This process ischaracterized in that it comprises in combination the following steps:

a. determination of the natural γ radiation spectrum emitted by thegeological formations in the form of a plurality of quantities relatedto this radiation and each of which is a function of the intensity ofthis natural radiation in a given energy band,

b. alternative transmission of acoustic waves being propagated in thegeological formations essentially according to the longitudinal way ormode and acoustic waves being propagated essentially according to thetransversal way or mode,

c. successive determination of the travel time of these longitudinalwaves and these transversal waves between two receivers placed incontact with the geological formations, by producing two quantitiesrelated to this travel time for the longitudinal and for the transversalwaves respectively,

d. successive determination between these receivers of the attenuationof these longitudinal and these transversal waves by producing twoquantities representative of this attenuation for the longitudinal wavesand the transversal waves, respectively, and

e. elaboration on the basis of these quantities in the automatic systemof at least one resultant quantity representative of one characteristicof the formation, this quantity being obtained by a linear combinationof the quantities related to the γ radiation with the quantities relatedto the travel time of the longitudinal and transversal acoustic waves,and with the quantities related to the attenuations of these acousticwaves.

More particularly, a precise identification of the geological formationsis obtained according to the present invention by:

a γ spectrometry for obtaining accurate data concerning the claycontent, less accurate data relating to the content of carbonates andthe cationic exchange capacity, and not very accurate data relating tothe quartz content, and

acoustic meansurements which furnish by a comparison of the respectivetravel times of the waves being propagated essentially longitudinallyand of the waves being propagated essentially transversally, accuratedata relating to the quartz content, less accurate data relating to thecontent of carbonates and not very accurate data relating to the claycontent.

The knowledge of the clay content, obtained by γ spectrometry permitsthe determination, on the one hand, of the porosity of the geologicalformations based on the knowledge of the acoustic waves travel time(particularly the longitudinal waves) and, on the other hand, based onthe knowledge of the attenuation of the acoustic waves (particularly thetransversal waves), of the mobility of the fluids inside the geologicalformation and accordingly, of the permeability of the geologicalformations, which is the product of the mobility by the viscosity of thefluid impregnating the formations, the viscosity being assumed to beconstant with a rather good accuracy.

The error in the measurement of the permeability is in fact substantialonly in the cases where the formations at the place of measurement,consist of:

dry rocks saturated with gas at low depth, and

oil-wet rocks saturated with viscous oil at low depth.

In the first case, a gas flow will be observed and it will be apparentthat the presence of this gas will explain the apparently highpermeability shown by the measurements.

In the second case, the interpretation may be made with certainty if theformation exhibits a high porosity and a low clay content, a combinationwhich does not allow for the low permeability values indicated by themeasurements.

The case where a viscous oil impregnates a very porous or very clayishformation is both very unusual and of little economical interest.

A device usable for carrying out the process according to the presentinvention such as indicated hereinabove comprises a sonde which isconnected, by means of a cable equipped with electric conductors, to asurface apparatus for the automatic treatment of data being furnished bythe sonde, and this device is characterized in that the sonde comprisesin combination the following elements:

a. means for detecting the natural γ radiation emitted by theformations, these means furnishing electric signals which arerepresentative of the γ radiation emitted by the geological formations,each of these signals being a function of the intensity of this naturalradiation in a given energy band,

b. first acoustic wave emitting means for emitting or transmitting inthe geological formations acoustic waves propagated essentiallyaccording to the longitudinal mode,

c. second acoustic wave emitting or transmitting means for emitting inthe geological formations acoustic waves propagated essentiallyaccording to the transversal mode, the first and second emitting meansoperating alternatively,

d. at least two receivers of acoustic waves for delivering electricsignals in response to the reception of the acoustic waves,

e. means for determining the travel time and the attenuation betweenthese receivers of these acoustic waves, these means being connectedwith the receivers and adapted to deliver a first series of electricsignals representative of the attenuation of the acoustic waves, and

f. means for transmitting to the surface all of the electric signals.

The invention will be better understood and further advantages thereofmade apparent from the following description of an embodiment of theinvention, given by way of nonlimitative example and illustrated by theaccompanying drawings wherein:

FIG. 1 illustrates the assembly of the device according to theinvention, used for surveying formations traversed by a borehole;

FIG. 2 illustrates a particular embodiment of the sonde forming part ofthe equipment of the device shown in FIG. 1;

FIG. 3 represents schematically the aspect of the γ spectrum emitted bythe geological formations;

FIG. 4 represents a surface apparatus for the analysis of the γradiation spectrum emitted by the geological formations;

FIG. 5 represents an embodiment of a surface apparatus treating in realtime the measurements effected by the sonde of FIG. 2 in order todetermine the lithology of the geological formations;

FIG. 6 represents another embodiment of a surface apparatus treating inreal time the measurements effected by the sonde of FIG. 2 in order todetermine the lithology of the geological formations;

FIG. 7 illustrates a second particular embodiment of a measuring sondeallowing for determining simultaneously the lithology, porosity andpermeability of the geological formations;

FIG. 8 illustrates a particular embodiment of the piezoelectric emittersand receivers of the sonde represented in FIG. 7;

FIG. 9 represents schematically an electronic apparatus for measuring ofthe attenuation of an acoustic wave being propagated in the geologicalformations;

FIG. 10 illustrates the operation of the apparatus of FIG. 9;

FIG. 11 represents schematically an apparatus for measuring the transittime of an acoustic wave being propagated in the geological formations;

FIG. 12 illustrates the operation of the apparatus of FIG. 11, and

FIG. 13 represents an embodiment of a surface apparatus treating in realtime the measurements effected by the sonde illustrated in FIG. 7.

FIG. 1 illustrates schematically the entire inventive device forcarrying out the process proposed by the present invention designed forthe exploration of geological formations traversed by a borehole.

In this figure, reference numeral 1 designates geological formationstraversed by a borehole 2. Suspended from an operating cable 3, a sonde4 is displaced in the borehole. The cable 3 comprises a plurality ofelectrical conductors connected by conductors 5 to a surface apparatus6. This apparatus, which will be described in further detail hereinafteris designed for supplying the sonde 4 with electric energy and fortreating the data being delivered by the sonde through the conductors ofthe cable 3.

FIG. 2 represents schematically a first particular embodiment of thesonde 4 which permits the determination of the lithology of thegeological formations.

The body 7 of the sonde which has, for example, a diameter in the orderof 110 millimeters is rigid. It is hollowed out in the central partthereof in order to receive two articulated pads or guide blocks 8 and9. These pads or guide blocks are actuated by means of articulated arms10 which are controlled by any means known, for example by hydraulicdevices 11a and 11b, and these two pads or guide blocks are incontact--in the measuring position of the sonde--with two diametricallyopposite generatrices of the borehole 2. A device 11 which is connectedwith the hydraulic means 11a and 11b allows for measuring the spacing ofthe guide blocks and supplies electric pulses having a low frequency,for example 1 Hz, and being coded as function of the value of theaverage diameter of the borehole opposite the guide blocks. These pulsesare transmitted to the surface by a conductor (not shown) which isembedded in the cable 3 from which the sonde is suspended. As shown inFIG. 2, the guide block 9 has a purely mechanical role, and thedisplacement thereof in a direction perpendicular to the borehole axisis greater than that of the guide block 8.

The guide block 8 which, in the measuring position represented in FIG.2, comes into contact with the wall of the borehole, comprises a γlogging device. This device may be of any known type, but according tothe preferred embodiment of the present invention, it contains means forstabilizing the γ spectrum emitted by the geological formations. Thisdevice may, for example, be that described in the French patent2,077,483 filed on Jan. 20, 1970. This device comprises a scintillator12, for example of the type with sodium iodide crystal, which isassociated in a known manner with a photomultiplier PM1 γ and which hasa casing 13a made of a material absorbing the γ rays only to a smallextent such as aluminum. Under this assembly is placed a β ray detectorcomprising in a manner known per se a scintillator 14 associated with aphotomultiplier PM2β. Arranged between the scintillators 12 and 14 is anauxiliary source of γ rays designated with reference numeral 15 which isat the same time a source of γ rays, this source consisting for example,and preferably, of a 22 sodium (²² Na) pellet.

The scintillator 14 is shielded on the lateral wall thereof by a layer16 of a material which strongly absorbs the γ rays, such as lead, andthe auxiliary source 15 is separated from the scintillator 12 by a sheet13b of a material forming an optical screen between the twoscintillators and stopping those of the β rays which would be directedtoward the scintillator 12. This sheet may consist for example ofaluminum which does not materially absorb the γ pulses being emitted bythe auxiliary source 15. The thickness of this sheet will be chosen inaccordance with the intensity of the reference γ radiation, i.e., inaccordance with the activity of the source of 22 sodium (²² Na) emittingthis radiation.

The operation of this device, which has been described in detail in theFrench patent 2,077,483 and which is not the essential object of thepresent invention will not be further indicated herein.

It should only be noted that what is obtained iis the stabilization ofthe natural γ spectrum emitted by the formations by making use of the γradiation emitted by the auxiliary source 15, and this radiation isknown and identified in the spectrum by its time coincidence with a γradiation which is emitted simultaneously by the auxiliary source 15.

The photomultipliers PM1.sub.γ and PM2.sub.β are connected to anelectronic assembly 22a, contained for example in the body 7 of thesonde 4 (for the sake of greater clarity of this figure, the electricalconnections have not been shown).

This assembly which is connected to the surface by means of electricalconductors of the cable 3 comprises the electronic devices for thesupply of the photomultipliers and the electronic members for thetreatment of the data being delivered or furnished by thephotomultipliers prior to their transmission to the surface by theelectrical conductors of the cable 3. The data is transmitted to thesurface in the form of pulses of constant duration, for example 140μ andhaving an amplitude proportional to the energy of the radiationsreceived by the detectors.

The cable 3 which assures the connection between the sonde and thesurface is advantageously a cable that is of a type commerciallyavaiable containing several conductors. One of the conductors isutilized as ground conductor and may be common to all of the electronicapparatus contained in the sonde. Two other conductors serve for feedingthe sonde with energy, for example an alternating current of 220 volt at50 Hz, from which the direct current high voltage and low voltagenecessary for the operation of the sonde is produced. Another conductoris used for the transmission of the γ spectrometry signals from thesonde toward the surface, and the data furnished by the device 11 (FIG.2) can be transmitted by another conductor of the cable 3.

During one measuring interval, the sonde thus furnishes to the surfaceapparatus the following signals:

a. coded pulses representative of the diameter of the borehole at thelevel of the sonde, and

b. γ pulses of constant durations the amplitudes of which areproportional to the energy of the γ radiation emitted by the formations.

The lithology of the geological formations is obtained by solvingequations of the type ##EQU1## wherein X_(j) designates one of thefollowing data: the content in clays

the content in carbonates

the content in quartz

the cationic exchange capacity.

The coefficients α_(ij) and β_(j) are given coefficients varying withthe data X_(j) to be measured. One way of determining these coefficientswill be indicated hereunder.

The coefficients C_(i) are determined by the measurements effected bythe sonde. These coefficients are variables, functions of the nature ofthe formation or terrain representing the contents of n bands A, B, C, D. . . N judiciously chosen in the γ spectrum emitted by the geologicalformations as represented, for example, in FIG. 3 in the case where n=4.

In order not to complicate the surface apparatus excessively, thedetermination of the coefficients α_(ij) and β_(j) is made bystandardization or calibration from measurements carried out, forexample in the laboratory, on samples of terrain taken at the time ofthe drilling.

In a preferred embodiment for carrying out the present invention, thesesamples will be cored samples, but it is equally possible to useexcavated drilling materials if it is known with certainty from whichslope they come. Determined on the samples thus taken, for example inthe laboratory and with conventional methods, are the values of thequantities X_(j) (clay content, carbonate content, quartz content andcationic exchange capacity). When carrying out the logging, the valuesof the variables C_(i) measured which correspond to the samples takenare noted. The sequence of the values of the coefficients α_(ij) andβ_(j) is then determined by the conventional mathematical process ofmultiple linear regression in a manner well-known in the art.

The n variables C₁ to C_(n) represent the number of γ rays recordedduring the measurement time (for example during 30 seconds) and havingan energy comprised between conveniently selected ranges. In practice,the number n is low, comprised between 3 and 7, and the range of energyexplored is extended, for example from 0.1 to 3.5 MeV, and in general atleast comprised between 0.4 and 2.7 MeV, at least one of the energybands being centered advantageously on the value of 1.47 MeV.

FIG. 4 illustrates an apparatus for the elaboration of the variablesC_(i) in which the number of these variables has been limited to four.The γ pulses from the sonde are transmitted simultaneously by conductors101 to 105 to amplifiers 106 to 110. Applied to the second inputs of theamplifiers are fixed voltages transmitted by the conductors 111 to 115.These voltages are regulable by potentiometers 116 to 120. When anamplifier receives an electric pulse corresponding to a γ pulse, itprovides on its output terminal either a logical signal of the level 0when the amplitude of the pulse is lower than the value of the fixedvoltage that is applied to this amplifier on the second input terminalthereof, or a logical signal of the level 1 when the amplitude of thepulse is greater than the value of the fixed voltage. For each pulsecoming from the sonde, the group of amplifiers furnishes a coded signaltransmitted by the conductors 121 to 125 to a decoding device 126connected to four digital memories 127 to 130 by the conductors 131 to134. The decoding device 126 provides for the recording of the receivedsignal in the corresponding memory. In this manner, a pulse having forexample an amplitude comprised between the voltages furnished by thepotentiometers 118 and 119 which delimit the C band of the spectrumplaces the outputs of the amplifiers 106 to 110 in the configuration1-1-1-0-0. This combination, analyzed by the device 126 causes theincrease of a unit of the memory 129 corresponding to the C band of theγ spectrum (FIG. 3). The potentiometers 116 to 120 are regulable so asto be adapted to adjust the limits of the energy bands to the valuesbeing best suited.

FIG. 5 represents schematically a hybrid computer for treating in realtime the measurements effected by the sonde, i.e., being adapted toresolve the equations (1).

This computer comprises a memory C in which the C_(n) variables arerecorded, as previously indicated in connection with FIG. 4. Stored inthis memory are the variables C₁ to C₄, n having been chosen to be equalto 4 in this example. The coefficients α_(ij) determined bystandardization, which are constant coefficients, are stored in a memoryα. The memory C furnishes on four output terminals four electric signalsrepresentative of the variables C₁ to C₄. These signals are transmittedto a logarithmic decoder 137 which furnishes on its four outputterminals signals proportional to the logarithm of the variables C₁ toC₄. These signals are transmitted respectively by the conductors 138 to141 to the input of the amplifiers A₁ to A₄.

In the same manner, the coefficients α_(ij) are read by a logarithmicdecoder 146 which furnishes on its output terminals signals proportionalto the logarithm of these coefficients α_(ij) and which are appliedrespectively by the conductors 147 to 150 to the amplifiers A₁ to A₄.The amplifiers A₁ to A₄ furnish on their respective output terminalselectric signals proportional to the sum of the signals which areapplied on their input terminals, i.e. signals proportional to thelogarithm of the products α_(ij) ·C_(i). The signals delivered by eachamplifier are transmitted respectively by conductors 155 to 158 tofunction generators GF₁ to GF₄ which restore on their output terminalssignals proportional to the products α_(ij) C_(i). The signals furnishedby the generators GF₁ to GF₄ are transmitted directly by the conductors163 to 166 to an operational amplifier 169 which sums these signals anddelivers on its output terminal an electric signal proportional toΣα_(ij) C_(i), transmitted by a conductor 170 to an analog-digitalconverter 171 which transmits its output signal via the conductor 172 toan addressing circuit 173 which provides for the display or storing ofthe result obtained in a part j of a memory M, this part correspondingto the determined quantity X_(j) (this quantity consisting of datarelating to the lithology), and this result is obtained in addition tothe previously determined constant coefficient β_(j) stored in thememory M.

A clock H drives simultaneously the logarithmic decoders 137 and 146 andthe addressing circuit 173, thus allowing for the successive storing inthe memory M of the quantities X_(j) corresponding to the lithology ofthe logged formations.

The memory M is connected to a device for recording and/or visuallydisplaying the determined quantities X_(j), this device, which has notbeen shown in the figure, may be of any known type, such as magneticrecording, recording on punched tape, etc.

FIG. 6 illustrates another embodiment of a hybrid computer for treatingin real time the measurements obtained from the sonde. As in the case ofthe computer described in connection with FIG. 5, there is an identicalmemory C in which the variables C_(i) are recorded, this memory beingassociated with a logarithmic decoder 137 having one output terminal. Inthe memory α associated with a logarithmic decoder 146, all thecoefficients α_(ij) are stored. The two logarithmic decoders 137 and 146are driven by a clock H and, at each pulse of the clock received bymeans of the conductors 180 and 181, the logarithmic decoders deliversimultaneously signals proportional to the corresponding variable C_(i)and to the coefficients α_(ij) and which signals are transmitted by theconductors 182 and 183 to the input of an amplifiers A. This amplifierdelivers a signal proportional to the logarithm of the term α_(ij) C_(i)which is transmitted by a conductor 184 to a function generator GF whichdelivers a signal proportional to the product α_(ij) C_(i). This signalis applied by means of the conductor 170 to an analog-digital converter171 which furnishes on its output terminal a signal which is transmittedby the conductor 172 to an addressing circuit 173 driven by the clock Hby means of the conductor 185. The addressing circuit 173 serves forstoring the received signal in part j of the memory M corresponding tothe computed quantity X_(j). The hybrid computer described in FIG. 11 isof a simpler construction than that described in connection with FIG.10, but it requires a longer time for the determination of the values ofthe quantities X_(j).

It is understood that, instead of treating on the site the measurementsobtained from the sonde, it would also be possible to record the sameand the recorded measurements would thereafter be transmitted to atreatment center where they could be combined according to the processwhich has been described.

FIG. 7 shows a sonde for determining simultaneously the lithology, theporosity, and the permability of geological formations. This sonde iscomposed of a γ logging device identical to that described with respectto FIG. 2, combined with an acoustic logging device as described below.

The pad or guide block 8 is equipped at the lower part thereof with twoacoustic receivers 17 and 18 and, at the upper part thereof, with anemitter or transmitter 19 of acoustic waves. Mounted on the upper partof the sonde body is an emitter or transmitter 20 of acoustic waves. Thetransmitters 19 and 20 and the receivers 17, 18 are disposed in the samediametric plane of the borehole. Only the transmitter 20 is not incontact with the wall of the borehole, whereas the transmitter 19 andthe receivers 17 and 18 carried by the guide block 8 are in contact withthe wall of the borehole 2 in the measuring position of the sonde.

The transmitter 20 is designed for generating waves propagatedessentially in the longitudinal way or mode. For this reason it isplaced slightly recessed in the body of the sonde so as to maintain athickness of several centimeters of drilling mud between the formationand this transmitter. The presence of this liquid interface stops thewaves being propagated in the transversal way or mode and allows onlyfor the propagation of longitudinal waves. According to a preferredembodiment, the transmission frequency is chosen higher than 20 kHz andis preferably in the range between 20 to 80 kHz, and, for technologicalreasons, a transmitter of the magnetostrictive type will be preferablyemployed.

The transmitter 19 is designed for generating waves similar totransversal waves. For this purpose and for technological reasons, it isof the piezoelectric type and operates, for example, at a frequency atleast equal to 80 kHz, and preferably in the range between 80 and 25kHz. It is applied against the formation without interposition of liquidfilm. Experience has shown that, under these conditions, the wave trainof the greater propagation velocity at the interface well formation andwhich will correspond to the first arrival of energy which will besolely taken into account at the receivers, is similar to a train oftransversal waves.

for each transmitter the power is such that, at each receiver, thesignal-to-noise ratio will permit the obtaining of accuratemeasurements. Since the attenuation of the acoustic waves during theirtravel in the geological formations is exponential, it is desirable thatthe receivers and transmitters be disposed as close to each other aspossible. Nevertheless, there is a limit since the spacing betweenreceivers must be sufficient in order that the measurements be possible,and the spacing between transmitters and receivers must be such that acertain number of static acoustic phenomena with respect to themeasurements which are produced particularly at the contact zone ofpiezoelectric transmitter with the formation be completely attenuated byabsorption in the geological formations. It has been found that theserequirements are met for a minimum distance between receivers in theorder of one wave length, and between transmitters and receivers in theorder of about ten wave lengths of the acoustic vibrations beingemitted.

In this embodiment, the distance between the receivers 17 and 18 is inthe range between 5 and 10 centimeters; the distance between thereceiver 17 and the transmitter 19 is in the range between 40 and 80centimeters, and the distance between the receiver 17 and thetransmitter 20 is chosen to be in the range between 60 and 150centimeters.

The transmitters and the receivers are supplied with electric energy bya feeding block schematically indicated with reference numeral 21 andhoused, for example, in the body of the sonde. The feeding block 21,which is of a conventional type, will not be described in detail herein.It furnishes to the various elements the electric energy with suitablevoltages and intensities. The signals delivered by the receivers 17 and18 are transmitted to an electronic group 22b which is housed, forexample, in the body of the sonde. These signals are treated by thegroup 22b--which will be described in further detail hereinbelow--andthe data transmitted to the surface by conductors of the cable 3.

FIG. 8 represents, at an enlarged scale, a particular embodiment of thepiezoelectric transmitters and receivers of the sonde placed in contactwith the geological formations. Each of these is composed of a feelinghead 24 housed in a cavity 25 which is disposed in the guide block 8 ofthe sonde. The feeling head 24 may be displaced in the cavity 25perpendicularly to the axis of the borehole 2 due to the action of anyknown means that may be operated by remote control, such as, forexample, elastic means, hydraulic means, or means with springs, beingplaced between the guide block and the end of the feeding head situatedat the inside of the cavity 25. The displacement of the feeling head 24may be on the order of several centimeters for reasons which will beexplained further hereunder.

The feeling head 24 which has, for example, an essentially cylindricalconfiguration has an essentially conical outer end or contact piece 27whose apex terminates in a spherical cap. The feeling head 24 consistsof a rigid, for example metallic, material adapted to transmit theacoustic vibrations. Disposed in the body of the feeling head 24 is afluid-tight housing 23a into which is placed a piezoelectric element 23which latter is sensitive to the acoustic vibrations and which is incontact with the walls of this housing. This element 23 may consist, forexample, of a stack of piezoelectric disks, made for example from bariumtitanate (Ba Ti O₄) whose axis of rotation which extends essentiallythrough the apex of the contact piece 27 and is perpendicular to theaxis of the borehole 2. The sensitive element 23 is provided for in sucha manner that, upon receiving an electric pulse, it vibrates preferablyin a direction perpendicular to the surface of the geological formationwith which it is in contact by means of the feeling head 24. Electricconductors 23b transmit the electrical signals delivered by thesensitive element 23, or transmitted thereto, depending upon whether thedevice operates as receiver or as transmitter.

The device operates in the following manner:

When the sonde has been placed into the borehole 2 at the desired depth,the guide blocks or pads are spaced from the body of the sonde untilthey come into contact with the mud cake 2a which covers the wall of theborehole. Upon the action of elastic means (not shown), the feeling headpenetrates into the cake until the end of the contact piece 27 comesinto contact with the geological formations. During the displacements ofthe sonde, when the measurements are made, the elastic means hold thecontact piece 27 constantly in contact with the geological formations.In the case where the device operates as a receiver, the acoustic wavespropagated in the geological formations are transmitted to the sensitiveelement 23 by the feeling head 24. The sensitive element 23 which isthen subjected to vibrations delivers an electric signal representativeof these waves, this signal then being transmitted by the conductors 23bto the electronic apparatus with which the sonde is equipped. In thecase where the device operates as a transmitter, the electric signaltransmitted by the conductors 23b causes the sensitive element 23 tovibrate, and these vibrations are transmitted to the geologicalformations by means of the contact piece 27 of the feeling head 24.

It is understood that the transmitters and receivers are separated fromeach other by means one of which is schematically indicated withreference numeral 8a in FIG. 8 and consists of a material absorbing thevibrations transmitted to the guide block 8.

The emitters 19 and 20 operate alternatively at a low recurrencefrequency in the range between 5 and 50 Hz, for example. The acousticwaves emitted by the piezoelectric transmitter 19 are propagatedpractically exclusively in the geological formations the first energyarrivals reaching successively the receivers 17 and 18 are similar towaves of the transversal type. The acoustic waves emitted by themagnetostrictive transmitter 20 are transmitted through the mud to thegeological formations in which they are propagated, and the first energyarrivals of the longitudinal wave type reach successively the receivers17 and 18.

Upon receiving of acoustic signals from any one of the transmitters, thereceivers deliver electric signals which are transmitted to theelectronic group 22b (FIG. 7) which generates for each of the waves asignal representative of the travel time in the geological formationsbetween the two transmitters the difference of the times of arrival ofan acoustic wave at each of the receivers and a signal representative ofthe attenuation of the wave between the two receivers (proportional tothe ratio of the amplitudes of the first electric oscillation deliversby each receiver).

FIG. 9 illustrates schematically the part of the electronic group 22serving for measuring the attenuation between the receivers 17 and 18 ofan acoustic wave transmitted from one or the other of the transmitters.

FIG. 10 represents the different electric signals illustrating theoperation of the device of FIG. 9.

An acoustic signal which is propagated in the geological formations bythe transmitter 20 or 19 reaches successively the receivers 17 and 18 atthe respective instants t₁ and t₁ +δt. The receiver 17 delivers anelectric signal S₁ shown in FIG. 10. This signal is an oscillatorysignal whose first half-cycle has an amplitude a₁. Upon receiving thesame acoustic signal, after a time interval δt, the receivers 18delivers an electric signal S₂ (FIG. 10) whose first half-cycle has anamplitude A₂. The electric signals S₁ and S₂ are transmittedrespectively by the conductors 28 and 29 to blocking-lengthening devices30 and 31. These devices pick up the first oscillations of the signalsS₁ and S₂, lengthen them respectively by a duration τ₁ and τ₂, anddeliver on the respective output terminals the electric signals T₁ andT₂ (FIG. 10).

The signal T₁ furnished by the device 30 is transmitted by the conductor32 to a polarity reverser 33 which delivers thereupon an electric signal-T₁ transmitted by a conductor 34 to a first input of an analog gate 35.The signal T₁ is simultaneously transmitted by the conductor 36 to asaturated amplifier 37. This amplifier 37 which operates in an on-offfashion delivers at its output a signal in the form of a rectangularwave U₁ (FIG. 10) having a fixed amplitude when it receives a controlsignal T₁. In this manner, the signal U₁ exists simultaneously with thesignal T₁, but its amplitude is independent of the amplitude of thecontrol signal T₁ which has initiated its generation. The signal U₁,furnished by the amplifier 37, is transmitted by the conductor 38 to afirst input of an AND type gate 39.

In the same manner, the signal T₂ delivered by the device 31 istransmitted by the conductor 40 to a first input of an analog gate 41and by the conductor 42 to a saturated amplifier 43 which, uponreceiving the control signal T₂, furnishes a signal U₂ (FIG. 10) of thesame amplitude as the signal U₁. The amplifier 43 is connected by theconductor 44 to the AND-type gate 39 and the signal U₂ is transmitted tothe second input of this AND gate. The AND gate generates a signal V(FIG. 10) having a constant amplitude when its receives simultaneouslyon the two inputs thereof the signals U₁ and U₂. As shown in FIG. 10,the signal V in the form of a rectangular wave exists from the instantt₁ +δt until the t₁ +τ₁, thus having a duration of τ₁ -δt. The output ofthe AND gate 39 is connected by way of the conductors 45 and 46 with theanalog gates 35 and 41, respectively. The analog gate 35 passes thesignal -T₁ issuing from the polarity reverser 33 only when it receivessimultaneously the control signal V by the conductor 45. In the samemanner, the analog gate 41 passes the signal T₂ issuing from theblocking-lengthening device 31 only when it receives simultaneously thecontrol signal V by the conductor 46. The analog gates 35 then deliverrespectively the electric signals W₁ and W₂ (FIG. 10) transmitted by theconductors 47 and 48 to the logarithmic amplifiers 49 and 50 whichfurnish on the output terminals thereof signals whose amplitude isproportional to the logarithm of the amplitudes of the signals W₁ andW₂, respectively. These logarithmic amplifiers are connected by means ofconductors 51 and 52 to a summing device 53 which furnishes an electricsignal X (FIG. 10) whose amplitude is proportional to the logarithm ofthe ratio A₂ /A₁.

The summing device 53 is connected by the conductor 54 with alengthening device 55 which delivers at the output thereof a signal L inthe form of a rectangular wave having the amplitude k Log (A₂ /A₁) andhaving a calibrated duration T. This predetermined duration is chosen tobe sufficient so as to ensure correct transmission of the signal L tothe surface by the cable, i.e., such that at the arrival of the signalon the surface there remains a portion of the signal which is notaffected by the transmission.

It is to be understood, as has already been indicated previously herein,that the transmitters 19 and 20 operate alternatively and the electronicapparatus of FIG. 9 successively delivers a signal L and a signal L'being representative of the attenuation of the transversal and thelongitudinal waves that are successively emitted by the twotransmitters. The signal L and L' which have calibrated durations, forexample in the order of 120μ, are transmitted by the same conductor ofthe cable 3, and since the recurrence frequency of operation of thetransmitters 19 and 20 is low (5 to 50 Hz), there is no interferencewhatsoever possible between the signals L and L' which are delivered bythe electronic apparatus substantially at the operating frequency of thetransmitters.

FIG. 11 illustrates schematically the electronic apparatus contained inthe sonde for measuring the travel time of an acoustic wave between thereceivers 17 and 18. FIG. 12 represents the signals produced during theoperation of the apparatus illustrated in FIG. 11.

As has already been indicated, an acoustic signal which is propagated inthe geological formations by the transmitter 19 or 20, reachessuccessively the receivers 17 and 18 at the respective instants t₁ andt₁ +δt, and the receivers deliver respectively the electric signals S₁and S₂ (FIG. 12). The signals S₁ and S₂ are transmitted respectively tomonostable or one-shot multivibrators 56 and 57 by the conductors 58 and59. Upon receiving of the signal S₁, the monostable multivibrator 56delivers a signal R₁ (FIG. 12) in the form of a rectangular wave, havinga constant amplitude and a calibrated duration D₁. This signal istransmitted by the conductor 60 to an input of a first device 61 whichis an AND-type gate, and by means of the conductor 62 to an input of asecond device 63 which is also an AND-type gate.

Upon receiving a signal S₂, the monostable multivibrator 57 delivers asignal R₂ (FIG. 12) in the form of a rectangular wave having a constantamplitude and calibrated duration D₂. This signal is transmitted by theconductor 64 to a logical device 65 which returns on one of its outputsthe signal R₂ having been transmitted by the cable 66 to a second inputof the AND gate 63. On the second output terminal thereof, the device 65furnishes a signal R₂ (FIG. 12) that is complementary of the signal R₂and transmitted by the conductor 67 to the second input terminal of theAND gate 61. The latter delivers a signal N (FIG. 12) when it receivessimultaneously the signals R₁ and R₂, i.e.,--as is apparent from FIG.12, a signal having a duration δt. The signal N transmitted by theconductor 68 actuates an analog gate 69 which connects a high-voltagesource HT to time-amplitude conversion circuit 70. The circuit 70comprises, for example, a capacitor C charged through a resistor R whosetime constant RC is much higher than δt. Closing of the analog gate bythe signal N produces the charge of the capacitor C during a timeinterval δt. At the end of the charge, the electric voltage at theterminals of the capacitor C reaches a value proportional to δt, a valuewhich it retains as long as the capacitor C is not discharged.

A reading amplifier 71 is connected to the output of the time-amplitudeconversion device 70 by a conductor 72 and delivers a signal M (FIG. 12)which is transmitted by a conductor 73 to an analog gate 74. The gate 74is connected by a conductor 75 to the output of the AND gate 63. Thelatter delivers a signal P (FIG. 12) in the form of a rectangular wavewhen it receives simultaneously by way of the conductors 62 and 66 thesignals R₁ and R₂, in other words, a signal beginning at the instant t₁+δt and having a duration of D₁ -δt, during which the signals R₁ and R₂exist simultaneously. The signal P actuates the analog gate 74 whichthen passes the signal M between the instants t+δt and t₁ +D, thusdelivering the signal Q (FIG. 12) having an amplitude proportional toδt. The signal Q is transmitted by the conductor 76 to a lengtheningdevice 77 which transforms the signal Q into a signal Δ having anamplitude k St and a predetermined duration D chosen in such a manner toensure a correct transmission of the signal Δ to the surface. The signalΔ furnished by the lengthening device 77 is transmitted to the surfaceby a conductor of the cable 3. The signal P furnished by the gate 63 istransmitted by the conductor 78 to a first input of a device 79 which isan OR-type gate and receives on the second input thereof the signal Δdelivered by the lengthening device 77 and transmitted by the conductor80. At the reception of one of these two signals, the OR gate 79delivers on its output terminal a signal transmitted by the conductor 81to a circuit 82 adapted to control the discharge of the capacitor C. Thesignal furnished by the gate 79 places the circuit 82 in operatingcondition. When this signal disappears, the circuit 82 delivers acontrol pulse transmitted by the conductor 83 for actuating an analoggate 84 that short-circuits the capacitor C and causes the abruptdischarge thereof.

It is understood as has previously been indicated herein that thetransmitters 19 and 20 operate alternatively and that the electronicapparatus of FIG. 11 successively delivers a signal Δ and a signal Δ'representative of the travel time in the geological formations betweenthe receivers 17 and 18 of the transversal and the longitudinal wavesemitted successively by the two transmitters. The signals Δ and Δ' whichhave calibrated durations, for example on the order of μs, aretransmitted to the surface by the same conductor of the cable 3, andsince the operation recurrence frequency of the transmitters 19 and 20is low (5 to 50 Hz), there is no interference whatsoever possiblebetween the signals Δ and Δ' which are delivered by the electronicapparatus substantially at the operating frequency of the transmitters.

In the embodiment described hereinabove, the first half-cycle of eachsignal delivered by the receivers has been used. Obviously it would bepossible to use, instead of the first half-cycle, a predeterminedhalf-cycle of each of the signals so as to obtain in the same manner thetravel times and the attenuations of the longitudinal and transversalacoustic waves.

The cable 3 which assures the connection between the sonde and thesurface is advantageously a cable of a commercially available typecontaining seven conductors. One of the conductors is used as groundconductor and may be common to all of the electronic apparatus containedin the sonde. Two other conductors allow for feeding or supplying thesonde with electric energy for example and alternating current of 220volt at 50 Hz from which there is produced the high-voltage andlow-voltage direct current necessary for the operation of the sonde.Another conductor is utilized for the transmission of the γ spectrometrysignals from the sonde toward the surface, the signals Δ, Δ', and L,L'corresponding to the acoustic measurements being transmitted by twoseparate conductors, while the data being delivered by the diametricmeasuring device 11 (FIG. 7) may be transmitted by one of the twoconductors being utilized for the transmission of acoustic data, orpossible, by the remaining conductor.

Besides the coded pulses representative of the diameter of the boreholeat the level of the sonde and the γ pulses having constant durationswhose amplitudes are proportional to the energy of the γ radiationemitted by the formations, the sonde also delivers to the surfaceapparatus, during one measuring interval, the following signals:

a. pulses having constant durations whose amplitudes are representativeof the travel time between the two receivers of the acoustic waves beingpropagated in the geological formations, and

b. pulses having constant durations whose amplitudes characterize theattentuation of the acoustic waves being propagated in the formations.

The mobility of the fluids at the interior of the geological formations,the lithology, and the porosity of the geological formations areobtained by resolving equations of the type ##EQU2## in which X_(j)designates one of the following quantities; the clay content

the content in carbonates

the quartz content

the cationic exchange capacity

the porosity

the mobility

As indicated hereinabove, the coefficients α_(ij) and β_(j) are givencoefficients varying with the data X_(j) to be measured. One way ofdetermining these coefficients will be indicated hereinafter.

The coefficients C_(i) are determined by the measurements that areeffected by the sonde. These coefficients which are variables andfunctions of the nature of the terrain, comprise

1. the coefficients C₁ to C_(n) representing the contents of n bands A,B, C, D . . . N judiciously chosen in the γ spectrum transmitted fromthe geological formations, as represented for example in FIG. 3 in thecase where n=4;

2. the coefficients C_(n+1) and C_(n+2) which are respectively theaverage values of the travel times Δ and Δ' between the receivers,transversal and longitudinal acoustic waves, these average values beingindicated as Δ_(m) and Δ'_(m) ; and

3. in the case where the measured quantity is the mobility of the fluidinside the geological formation, with X_(j) then being the logarithm ofthe mobility that is sought, equation (2) comprises two supplementaryvariables C_(i), namely C_(n+3) and c_(n+4), representing respectivelythe average value of the logarithm of the ratio of the amplitudes of thefirst half-cycles of the signals delivered by the receivers, theseaverage values being indicated as L_(m) and L'_(m). It is understoodthat, in solving the equation, two corresponding coefficients α_(ij) arethen introduced (α.sub.(n+3).sbsb.j and α.sub.(n+4).sbsb.ij).

In order not to complicate the surface apparatus excessively, thedetermination of the coefficients α_(ij) and β_(j) is made bystandardization or calibration on the basis of measurements, carried outfor example in the laboratory, on ground samples taken during thedrilling operation. In a preferred embodiment, these samples are coredsamples, but it is equally possible to use excavated drilling materials,if it is known with certainty from which slope they come.

The number of terrain samples necessary may be small since it issufficient to have at least one sample representing each of the types ofgeological formations traversed by the borehole. On the basis of thesesamples taken in this manner, a determination is made, for example inthe laboratory and with conventional methods, of the values of thequantities X_(j) (clay content, carbonate content, quartz content,cationic exchange capacity, porosity, and mobility). During the loggingoperation, there is noted the values of the variables C_(i) measurementscorresponding to the samples taken. The series of the values of thecoefficients α_(ij) and β_(j) is thereafter determined by theconventional mathematical process of multiple linear regression which iswell-known in the art.

The first n variables C₁ to C_(n) represent the number of γ raysrecorded during the measuring time (for example during 30 seconds) andhaving an energy comprised between predetermined limits. In practice,the number n will be small, comprised between 3 and 7, and the energyrange explored will be large, for example from 0.1 to 3.5 MeV, andgenerally at least between 0.4 and 2.7 MeV. The elaboration orproduction of the variables C_(i) is carried out as indicatedhereinabove in connection with FIG. 4.

The variables C_(n+1) to C_(n+4) represent, as indicated above, theaverage values Δ_(m), Δ'_(m), L_(m), L'_(m) or the pulses Δ, Δ', L, L'which may be established during the time necessary for the recording,for example 30 seconds, a time period during which there is obtained foran operating recurrence frequency of the acoustic transmitters comprisedbetween 5 and 50 Hz, a number of measurements between 75 and 750 foreach of the values Δ, Δ', L and L', as defined previously. The averageis obtained, for example, by adding up the values of each measurement infour digital memories.

A clock for synchronizing the measurements relative to the γ radiationwith the acoustic measurements is utilized for determining theseaverages. This clock may be contained in the sonde, or in the surfaceapparatus, and it assures moreover the separation of the pulses Δ, Δ',and L, L' and the pulses delivered by the diametric measuring device 11of the sonde being transmitted by two conductors of the cable 3.

The treatment of each series of pulses Δ, Δ', and L, L' is performed byan analog-digital conversion of the amplitude of the transmitted pulse.This may be obtained, for example, by an amplitude-time conversion withthe notation being operated, during the time interval proportional tothe amplitude of the pulse, by opening a gate through which pass veryshort pulses calibrated at high frequency (for example 20 MHz), andthese pulses are stored in a counter register and added to the contentsof the digital memory.

FIG. 13 represents schematically a hybrid computer of the type such asthe one described in connection with FIG. 5 and designed for treating inreal time the measurements obtained from the sonde described in FIG. 7,i.e., a hybrid computer adapted to solving the equations (2).

This computer comprises a memory C within which are recorded the C_(n+4)variables defined previously. Stored in this memory are the variables C₁to C₈, which have not been chosen to be equal to 4 in this example. Thecoefficients α_(ij) determined by standardization or calibration whichare constant coefficients are stored in a memory α. The supplementarycoefficients α_(ij), i.e., the coefficients α.sub.(n+3).sbsb.j andα.sub.(n+4).sbsb.j defined hereinabove and being necessary fordetermining the mobility logging are stored in the form of voltagesdelivered by two potentiometers 135 and 136 which allows for asimplified arrangement of the memory α.

The memory C delivers on eight output terminals eight electric signalsrepresentative of the variables C₁ to C₈. These signals are transmittedto a logarithmic decoder 137 which delivers on its eight outputterminals signals proportional to the logarithm of the variables C₁ toC₈. These signals are transmitted respectively by the conductors 138 to145 to the input of the amplifiers A₁ to A₈.

In the same fashion, the coefficients α_(ij) (from α_(ij) toα.sub.(n+2).sbsb.j) are read by a logarithmic decoder 146 which deliverson its output terminals signals proportional to the logarithm of thesecoefficients α_(ij), which are applied respectively by the conductors147 to 152 to the amplifiers A₁ to A₆. The voltages delivered by thepotentiometers 135 and 136 which are proportional to the logarithm ofthe supplementary coefficients necessary for determining the mobilityare transmitted respectively by the conductors 153 and 154 to theamplifiers A₇ and A₈.

The amplifiers A₁ to A₈ deliver on their respective output terminalselectric signals proportional to the sum of the signals which areapplied on their input terminals, i.e., signals proportional to thelogarithm of the products α_(ij) ·C_(i). The signals delivered by eachamplifier are respectively transmitted by conductors 155 to 162 tofunction generators GF₁ to GF₈ which restore on their output terminalssignals proportional to the products α_(ij) ·C_(i). The signalsfurnished by the generators GF₁ to GF₈ are transmitted directly by theconductors 163 to 168 to and 176 to 177 to an operational amplifier 169which sums these signals and delivers on its output terminal an electricsignal proportional to ##EQU3## transmitted by a conductor 170 to ananalog-digital converter 171 which transmits its output signal throughthe conductor 172 to an addressing circuit 173 which serves for storingthe result obtained in a part j of a memory M, this part correspondingto the determined quantity X_(j) (this quantity consisting of data onthe lithology, the porosity, or the mobility), and this result beingobtained in addition to the previously determined constant coefficientβ_(j) stored in the memory M.

A clock H simultaneously controls the logarithmic decoders 137 and 146and the addressing circuit 173, thus allowing for the successive storingin the memory M of the quantities X corresponding to the lithology andto the porosity of the logged formations. The clock H also controls twogates 174 and 175 which connect respectively the function generators GF₇and GF₈ with the amplifier 169 by way of the conductors 176 and 177solely during the determination of the quantity X_(j) corresponding tothe mobility (or to the permeability of the surveyed formations.

The memory M is connected to a recording device and/or to a displaydevice of the determined quantities X_(j), and this device (not shown inthe figure) may be of any known type (magnetic recording, recording onpunched tape, etc.)

It is additionally possible to utilize a hybrid computer having a systemarrangement identical to that described with reference to FIG. 6, inwhich the two coefficients α.sub.(n+3).sbsb.j and α.sub.(n+4).sbsb.jnecessary for determining the permeability are stored in the memory α.

Modifications may be applied to the present invention without departingfrom the spirit and scope thereof. For example, it is possible to useonly the data delivered by the acoustic device of the sonde illustratedin FIG. 7, particularly in order to determine the permeability of thenonclay content geological formations.

We Claim:
 1. A process for determining the characteristics of thegeological formations traversed by a bore hole, making use of ameasuring sonde in combination with an automatic system for thetreatment of the data delivered by the sonde, comprising the combinationof the following steps:a. determining the spectrum of the γ raysnaturally radiated from the geological formations in the form of aplurality of quantities associated to said radiation, each of whichdepends on the intensity of this natural radiation, in a given energyrange, b. alternately transmitting acoustic waves which propagateessentially according to the longitudinal way of propagation andacoustic waves having essentially a transversal way of propagation, c.successively determining the travel time of said longitudinal waves andof said transversal waves between two receivers placed in contact withthe geological formations, by producing two quantities which arefunctions of the respective travel times of said longitudinal waves andtransversal waves, d. successively determining, between said receivers,the attenuation of said longitudinal and of said transversal waves, byproducing two quantities representative of said attenuation respectivelyfor the longitudinal waves and for the transversal waves, and e.elaborating, on the basis of said quantities, in said automatic system,at least one resulting quantity, representative of a characteristic ofsaid formations, by linearly combining said quantities associated to theγ radiation with said quantities which are functions of the travel timeof the longitudinal and transversal waves and with said quantitiesrepresentative of the attenuations of said acoustic waves.
 2. A processfor determining the characteristics of the geological formationstraversed by a bore hole, comprising the combination of the followingsteps:a. determining the spectrum of the natural γ rays radiated fromthe geological formations in the form of a plurality of signalsassociated to said radiation, each of which depends on the intensity ofthis natural radiation in a given energy range, b. alternatelytransmitting acoustic waves propagating in the geological formationsessentially according to the longitudinal way of propagation andacoustic waves propagating essentially according to the transversal wayof propagation, c. successively determining the travel time of saidlongitudinal waves and of said transversal waves between two receiversplaced in contact with the geological formation, by producing twosignals related to the respective travel times of the longitudinal wavesand the transversal waves, d. successively determining between saidreceivers, the attenuation of said longitudinal waves and of saidtransversal waves, by producing two signals representative of saidattenuation respectively for the longitudinal waves and for thetransversal waves, e. recording said signals, and f. elaborating, on thebasis of said recorded signals, at least one resulting signal,representative of a characteristic of said formations, by linearlycombining said signals associated to the γ radiation with said signalsrelated to the travel time of said longitudinal and transversal waves,and with said signals representative of the attenuation of said acousticwaves.
 3. A device for determining the characteristics of the geologicalformations traversed by a bore hole, comprising a sonde connected to asurface apparatus through a cable provided with electric conductors,said sonde comprising the combination of:a. means for detecting thenatural γ radiation transmitted from the formations, said meansdelivering electric signals representative of the radiation emanatingfrom the geological formations, b. means for transmitting acousticwaves, adapted to transmit to the geological formations, acoustic waveshaving essentially a longitudinal way of propagation, c. means fortransmitting acoustic waves, adapted to transmit to the geologicalformations, acoustic waves having essentially a transversal way ofpropagation, d. means for alternately operating said first and secondtransmitting means, e. at least two acoustic wave receivers spacedvertically, adapted to deliver electric signals at the time of receptionof said acoustic waves, f. means for determining the travel time and thewaves attenuation between said receivers of said acoustic waves, saidmeans being connected to said receivers and adapted to deliver a firstseries of electric signals representative of the travel time and asecond series of electric signals representative of the attenuation ofsaid acoustic waves, and g. means for transmitting to the surface allthese electric signals.
 4. A device according to claim 3, wherein saidfirst and said second transmitting means transmit acoustic waves of ahigh frequency and said acoustic waves transmitted from said firsttransmitting means have a frequency different from that of the acousticwaves transmitted from said second transmitting means.
 5. A deviceaccording to claim 3, wherein said first transmitting means are adaptedto transmit acoustic waves whose frequency is at least 20 kHz.
 6. Adevice according to claim 3, wherein said first transmitting means areadapted to transmit acoustic waves whose frequency is in the range offrom 20 kHz to 80 kHz.
 7. A device according to claim 3, wherein saidtransmitting means are adapted to transmit acoustic waves whosefrequency is at least 80 kHz.
 8. A device according to claim 3, whereinsaid second transmitting means are adapted to transmit acoustic waveswhose frequency is in the range of from 80 kHz to 250 kHz.
 9. A deviceaccording to claim 3, wherein said first and second transmitting meansare operating alternately at a recurrence frequency between 5 and 100Hz.
 10. A device according to claim 9, wherein said recurrence frequencyis about 50 Hz.
 11. A device according to claim 3, wherein said firsttransmitting means, said second transmitting means and said receiversare placed in the same diametral plane of the sonde.
 12. A deviceaccording to claim 3, wherein said first transmitting means consist ofan acoustic transmitter of the magnetostrictive type.
 13. A deviceaccording to claim 3, wherein said second transmitting means consist ofan acoustic transmitter of the piezo-electric type.
 14. A deviceaccording to claim 3, wherein said sonde is provided with a movable padfor supporting and for positioning said means for detecting the γradiation emanating from the geological formations, said second acousticwaves transmitting means and said receivers are supported on the samemovable pad of the sonde, so as to come into contact with the wall ofthe bore hole along a generatrix thereof.
 15. A device according toclaim 3, wherein said receivers are at a distance from one another equalto a few times the wave length of the transmitted acoustic waves and thedistance between said receivers and second acoustic waves transmittingmeans is greater than ten wave lengths of the transmitted acousticwaves.
 16. A device according to claim 3, wherein said means formeasuring the attenuation of the acoustic waves comprise a firstblocking-lengthening element connected to a first of said receivers, asecond blocking-lengthening element connected to the second of saidreceivers, each of said elements delivering an electric signal ofpredetermined duration having an amplitude substantially proportional tothe peak amplitude of a predetermined half cycle of the electric signalssupplied by said receivers, this device also comprising a firstanalogical gate, a first amplifier and a polarity inverter, saidpolarity inverter connecting the first element on the one hand, to saidfirst analogical gate and, on the other hand, to said amplifierdelivering an electrical signal in the form of a rectangular wave ofsubstantially constant amplitude and of the same duration as that of thesignal delivered by said first blocking-lengthening element, a gate ofthe AND type having a first input connected to said first amplifier, thedevice further comprising a second analogical gate and a secondamplifier said second element being connected, on the one hand, directlyto said second analogical gate and, on the other hand, to said secondamplifier delivering an electric signal in the form of a rectangularwave of substantially constant amplitude having the same duration thanthe signal delivered by said second blocking-lengthening element, saidsecond AND gate having a second input connected to said amplifier, saidAND gate being connected to said first and second analogical gates andsupplying thereto a control signal when it simultaneously receives thesignals delivered by each one of said first and second amplifiers, saidanalogical gates giving passage to the signals supplied by theblocking-lengthening elements only at the time of reception of saidcontrol signal, logarithmic amplifiers respectively connected to saidfirst and second analogical gates and delivering signals substantiallyproportional to the logarithms of the amplitude of the signals suppliedby said blocking-lengthening element, a summing amplifier having aninput connected to said logarithmic amplifiers said summing amplifierdelivering at its output terminal a signal the amplitude of which issubstantially proportional to the logarithm of the ratio of theamplitudes of the electric signals supplied by said blocking-lengtheningelements, and transmitting means connected to said summing amplifier,said transmitting means comprising a pulse lengthener, adapted tocalibrate to a predetermined value the duration of the pulses suppliedby said summing amplifier.
 17. A device according to claim 16, whereineach of said blockinglengthening elements delivers a signalsubstantially proportional to the peak amplitude of one of the firstfive half-cycles of the electric signal delivered by each receiver. 18.A device according to claim 17, wherein each of said blockinglengtheningelements delivers a signal substantially proportional to the peakamplitude of the first half-cycle of the electric signal delivered byeach receiver.
 19. A device according to claim 3, wherein said means formeasuring the travel time of the acoustic waves comprises a firstone-shot multivibrator connected to said first receiver and a secondone-shot multivibrator connected to said second receiver, each of saidmultivibrators being adapted, when receiving a signal supplied by saidreceivers, to deliver, at its output terminal, a signal of calibratedamplitude and duration, said first multivibrator being connected to afirst and a second gate of the AND type, said second multivibrator beingconnected to a device which, when it receives a signal supplied by saidsecond multivibrator, delivers at one of its outputs at said second gatea signal identical to the received signal and delivers at its secondoutput terminal connected to said first gate, an electric signal only inthe absence of signal delivered by said second multivibrator, said firstgate delivering a control signal of a duration equal to the timeinterval during which signals are simultaneously received at its inputterminals, this time interval being substantially equal to the traveltime of the acoustic waves between the two receivers, said controlsignal actuating over its whole duration, an analogical gate connectingto a voltage source, a time-amplitude conversion device, the output ofwhich is connected to amplifying means delivering a signal whoseamplitude is proportional to said travel time, said amplifying meansbeing connected, through an analogical gate, controlled by the signaldelivered by said second AND gate, to transmitting means comprising adevice for lengthening the signal delivered by said amplifying means anda device for resetting to zero said time-amplitude conversion deviceactuated by the output signal of said second AND gate.
 20. A deviceaccording to claim 3, wherein the surface apparatus comprises means forrecording all the signals delivered by the sonde and means forelaborating in real time from said signals at least one resulting signalrepresentative of a characteristic of said formations, said means forelaborating being connected to said means for recording and linearlycombining said signals associated to the γ radiation, said signals beinga function of the travel time of the longitudinal and transversalacoustic waves and said signals being representative of the attenuationsof said acoustic waves.
 21. A device according to claim 20, wherein saidmeans for elaborating in real time comprise a first memory wherein arerecorded, in the form of signals, predetermined coefficients, a secondmemory wherein are recorded said group of signals delivered by thesonde, a group of means for treating said signals, connected to each ofsaid memories actuated by a clock to which they are connected, saidmeans being adapted to combine the signals of said group and to deliver,at their output terminal, at least one digital signal function of thereceived signals and representative of an information element concerninga characteristic of the geological formations, and a third memory,connected to said treating means, where is recorded said representativesignal.
 22. A process for .Iadd.quantitatively .Iaddend.determining.[.the characteristics.]. .Iadd.at least one of a lithological andpetrophysical characteristic .Iaddend.of the geological formationstraversed by a borehole, utilizing a measuring sonde which deliverssignals, and an automatic system for treating the signals delivered bythe sonde, said process comprising the steps of:a. determining values ofcoefficients depending on the characteristics of the geologicalformations and registering the values of the coefficients in theautomatic system, b. determining the spectrum of the γ rays naturallyradiated from the geological formations in the form of a plurality ofquantities associated to said radiation, each of which depends on theintensity of this natural radiation in a given predetermined energyrange of the spectrum, and c. elaborating on the basis of saidquantities in said automatic system at least one resulting quantityrepresentative of a characteristic of said formation by linearlycombining said quantities as a function of the values of saidcoefficients. .[.23. A device for determining the characteristics of thegeological formations traversed by a borehole, comprising a measuringsonde placed in the borehole and delivering electrical signals, asurface apparatus combined with said sonde for the automatic treatmentof the signals delivered by the sonde, said surface apparatus comprisingmeans for registering values of coefficients depending on thecharacteristics of the geological formations a cable having electricalconductors, said sonde comprising: a. means for detecting the natural γradiation emitted by the formations, and for producing electricalsignals representative of the γ radiation emitted by the geologicalformations each of the signals being a function of the intensity of thenatural radiation within a given predetermined energy range of thespectrum, and b. means for transmitting the electrical signals to thesurface apparatus..].
 24. A device for .Iadd.quantitatively.Iaddend.determining .[.the characteristics.]. .Iadd.at least one of alithological and petrophysical characteristic .Iaddend.of the geologicalformations traversed by a borehole, comprising a measuring sonde placedin the borehole and delivering electrical signals, a surface apparatuscombined with said sonde for the automatic treatment of the signalsdelivered by the sonde, values of coefficients depending on thecharacteristics of the geological formations being registered in theautomatic system, a cable having electrical conductors, said sondecomprising:a. means for detecting the natural γ radiation emitted by theformations and for producing electrical signals representative of the γradiation emitted by the geological formations, each of the signalsbeing a function of the intensity of the natural radiation within agiven predetermined energy range of the spectrum, and b. means fortransmitting the electrical signals to the surface apparatus, andwherein the surface apparatus comprises means for recording saidelectrical signals delivered by and transmitted form said sonde, andmeans for elaborating in real time from said electrical signals at leastone resulting signal representative of a characteristic of saidformations, said means for elaborating being connected to said recordingmeans and linearly combining said electrical signals associated to the γradiation.
 25. A device according to claim 24, wherein said means forelaborating in real time comprises .[.flat.]. .Iadd.first.Iaddend.memory means to which predetermined coefficients are suppliedand are recorded therein in the form of signals, second memory means inwhich said signals delivered by the sonde are recorded, clock meansdelivering a clock signal and connected to said first and second memorymeans, combining means connected to said first and second memory meansand responsive to a clock signal for combining the signals from saidfirst and second memory means and providing at least one digitalresultant signal which is representative of an information elementconcerning a characteristic of the geological formations, and thirdmemory means connected to said combining means for recording saidrepresentative resultant signal.
 26. A process for .Iadd.quantitatively.Iaddend.determining .[.the characteristics.]. .Iadd.at least one of alithological and petrophysical characteristic .Iaddend.of the geologicalformations traversed by a borehole, using a combination a measuringsonde which delivers signals and an automatic system for treating thesignals delivered by the sonde, said process comprising the steps of:a.determining values of coefficients depending on the characteristics ofthe geological formations and registering the values of thecoefficients, b. determining the spectrum of the natural γ rays radiatedfrom the geological formations in the form of a plurality of signalsassociated to said radiation, each of which depends on the intensity ofthis natural radiation in a given predetermined energy range of thespectrum, c. recording said signals, and d. elaborating on the basis ofsaid signals in said automatic system, at least one resulting signalrepresentative of a characteristic of said formation by linearlycombining said signals as a function of the values of said coefficients.27. A process for .Iadd.quantitatively .Iaddend.determining.[.characteristics.]. .Iadd.at least one of a lithological andpetrophysical characteristic .Iaddend.of a geological formationtraversed by a borehole, utilizing a measuring sonde including meansdelivering measuring signals, and a data processing system for treatingsaid measuring signals, this process comprising the steps of:a.determining from said measuring signals a plurality of quantitiesassociated to the natural γ radiation of the geological formations, eachof said quantities depending on the intensity of said natural radiationin a given predetermined energy range of the spectrum, b. determiningthe values of .Iadd.calibration .Iaddend.coefficients .[.depending on atleast one characteristic of the geological formation.]., and c.elaborating through said data processing system, on the basis of saidmeasured quantities and of said coefficients, a .[.plurality of.].resulting .[.quantities.]. .Iadd.quantity .Iaddend.representative ofsaid at least one characteristic of said formation, said resulting.[.quantities.]. .Iadd.quantity .Iaddend.being derived from saidmeasuring quantities through .[.a plurality of.]. .Iadd.at least one.Iaddend.linear .[.relationships.]. .Iadd.relationship.Iaddend.including a plurality of coefficients having said valuesdetermined in step (b). .Iadd.
 28. A process according to claim 26wherein the values of the coefficients determined in step (b) depend onat least one lithology characteristic. .Iaddend..Iadd.
 29. A processaccording to claim 28, wherein the at least one lithology characteristicincludes the clay content of the geological formation. .Iaddend..Iadd.30. A process for determining the clay content of a geological formationtraversed by a borehole, utilizing a measuring sonde including meansdelivering measuring signals, and a data processing system for treatingsaid measuring signals, this process comprising the steps of: (a)determining from said measuring signals a plurality of quantitiesassociated to the natural γ radiation of the geological formations, eachof said quantities depending on the intensity of said natural radiationin a given predetermined energy range of the spectrum, (b) determiningthe values of calibration coefficients, and (c) elaborating through saiddata processing system, on the basis of said measured quantities and ofsaid coefficients, resulting quantity representative of the clay contentof said formation, said resulting quantity being derived from saidmeasured quantities through a linear relationship including a pluralityof coefficients having said values determined in step (b)..Iaddend..Iadd.
 31. A process according to claim 30, wherein saidplurality of quantities determined in step (a) depend on the intensityof the natural radiation in a plurality of substantially contiguousenergy ranges of the γ radiation spectrum. .Iaddend.